It has long been known to have silicon melted down and crystallized in quartz molds to produce multi-crystalline silicon blocks for subsequent processing in photovoltaics. During the crystallizing and cooling of the silicon, heat is removed by the dissipation thereof to the outer walls of the mold and the surface of the silicon. A drawback of this process is that the silicon often cools non-uniformly, thus producing intensive thermal stresses which promote dislocation and the multiplication thereof and produce cracks in the solidified block-shaped silicon. The non-uniform cooling also promotes back diffusion of foreign matters, especially metals, from marginal regions into the interior of the block-shaped silicon. Both the dislocations and the back-diffused foreign matters act as recombination centers and reduce the photovoltaic efficiency of solar cells.